1. Field of the Invention
The present invention relates to multiple radio interfaces, and more particularly, to a method for reducing interference during communications between a Mobile Station (MS) having both a Wireless Fidelity (Wi-Fi) interface and an Institute of Electrical and Electronics Engineers (IEEE) 802.16m interface and each system, when the MS communicates with a plurality of systems in a network where a Wi-Fi wireless network system is co-located with an IEEE 802.16m system called an International Mobile Telecommunications-Advanced (IMT-Advanced) system.
2. Discussion of the Related Art
Wireless Local Area Network (WLAN) is a network environment in which a LAN service is provided to a wireless terminal equipped with a WLAN card, such as a Personal Digital Assistant (PDA) or a laptop computer, through an Access Point (AP) corresponding to a wired LAN hub. Briefly, WLAN may be regarded as a system that substitutes a radio link between an AP and a Network Interface Card (NIC) like a WLAN card for a wired link between a hub and a user terminal in a legacy Ethernet system. Owing to no need for wiring to a wireless terminal, WLAN advantageously facilitates relocation of the wireless terminal and network configuration and extension, and enables communications of the wireless terminal during movement. Despite these benefits, WLAN offers a relatively low data rate, experiences unstable signal quality inherent to the nature of radio channels, and causes signal interference.
FIG. 1 illustrates an exemplary WLAN configuration.
Referring to FIG. 1, two types of network configurations are defined for WLAN depending on inclusion of APs. A WLAN configuration including APs is called an infrastructure network, whereas a WLAN configuration without APs is called an ad-hoc network. The service area of an AP is defined as a Basic Service Area (BSA), and an AP and wireless terminals connected to the AP form a Basic Service Set (BSS). A service provided to a wireless terminal through the connection to the AP is referred to as a Station Service (SS). The SS also covers a service between wireless terminals in the ad-hoc network. As illustrated in FIG. 1, service areas (BSAs) may overlap with each other. Two or more APs may enable communications between wireless terminals connected to the APs through interworking between the APs. In this case, the interconnections of the APs form a Distribution System (DS). A service provided through the DS is called a Distribution System Service (DSS) and an area to which the DSS is available is called an Extended Service Area (ESA). All wireless terminals that receive the DSS and APs within the ESA are collectively called an Extended Service Set (ESS).
FIG. 2A illustrates a Medium Access Control (MAC) architecture in IEEE 802.11.
Referring to FIG. 2A, the IEEE 802.11 MAC layer is based on a contention-based Distributed Coordination Function (DCF), added with a non-contention-based Point Coordination Function (PCF).
The DCF is a basic medium access mechanism of the IEEE 802.11 MAC layer, which adopts Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The DCF is implemented in all STAtions (STAs), for application to an Independent BSS (IBSS) or an infrastructure network. Before signal transmission, an STA senses a channel to determine whether another STA is transmitting a signal on the channel, and continues to sense the channel, while subtracting a channel idle time from a random backoff time. If the channel is idle until the backoff time reaches zero, the STA initiates a transmission. Otherwise, the STA attempts a transmission using the remaining backoff time in a Contention Period (CP) after the current transmission ends. Meanwhile, if the STA succeeds in acquiring the channel in the backoff procedure, it solves a hidden terminal problem by exchanging short control frames, i.e. a Request To Send (RTS) frame and a Clear To Send (CTS) frame.
The PCF is an optional medium access mechanism of the IEEE 802.11 MAC layer, applicable to an infrastructure network only. The PCF uses a polling scheme in which a Point Coordinator (PC) of an AP in a BSS polls STAs for transmissions. The PCF is implemented by access priority mechanism-based virtual carrier sensing. That is, the PCF uses a beacon management frame to set a Network Allocation Vector (NAV) and control medium access for an MS. Both the DCF and the PCF may be used for the same BSS. When a PC operates in a BSS, the DCF and the PCF alternate with each other, such that a CP alternates with a Contention-Free Period (CFP).
FIG. 2B illustrates an exemplary format of a management frame.
Referring to FIG. 2B, a Beacon frame, a Probe Request/Response frame, an Authentication Request/Response frame, an Association Request/Response frame, or the like is included in a frame body of a management frame. These frames are used for a wireless terminal to access an AP.
A Wi-Fi beacon message is periodically broadcast from a Wi-Fi AP. An STA may measure the channel status between it and the AP from the beacon message and estimate a Target Beacon Transmission Time (TBTT) using beacon interval information included in the beacon message. Generally, the STA measures the channel status by receiving a plurality of beacon messages. Besides, the beacon message includes information about capabilities supported by the AP (e.g. infrastructure mode/ad-hoc network mode, information indicating whether the PCF is supported, information indicating whether data encryption is supported, a Service Set Identifier (SSID), etc.). Hence, the beacon message is the first message that the STA receives to initiate communications with the AP.
To avoid contention for data transmission, the STA senses a channel and if the medium is sensed as idle, it takes a data transmission opportunity. When the STA receives an ACKnowledgment (ACK) for the transmitted data, this confirms successful data transmission. If the STA fails to receive the ACK message, it retransmits the data a backoff time later. The STA may perform a process for sensing that another STA is occupying the channel by transmitting a simple RTS/CTS control message before the data transmission.
FIG. 3 illustrates a frame structure under consideration for an IEEE 802.16m system.
Referring to FIG. 3, each 20-ms superframe is divided into four 5-ms frames, each frame including eight subframes. The subframes are allocated for DownLink (DL) and UpLink (UL) transmission. In the illustrated case of FIG. 3, DL and UL subframes are allocated at a ratio of 5:3 in a frame. One subframe may include six symbols, each symbol being 617 μs in duration. A SuperFrame Header (SFH) is transmitted every 20 ms, carrying cell-specific system information on a Secondary Broadcast CHannel (SBCH) and common system information on a Primary Broadcast CHannel (PBCH). Compared to an IEEE 802.16e broadband wireless access system, the IEEE 802.16m system may transmit a MAP (A-MAP) at a variable position. Since both the downlink and the uplink can be described with a MAP in a frame, a data transmission delay may be reduced.
FIG. 4 illustrates the frequency spectrum of a co-located co-existence environment.
Referring to FIG. 4, because the IEEE 802.16m system uses a frequency band adjacent to the Industrial, Scientific, and Medical (ISM) band of Bluetooth (BT) and Wi-Fi systems, severe interference occurs if a plurality of radio technologies are used independently or simultaneously in an STA. For example, the IEEE 802.16m system and the Wi-Fi system operate independently. Hence, when an MS receives data from the IEEE 802.16m system, it may occur that transmission of a Wi-Fi data packet is blocked.
In this context, to minimize interference between systems, a legacy Worldwide Interoperability for Microwave Access (WiMAX) system allows an MS to attempt to communicate with a Wi-Fi network during a sleep interval and to resume communications with a WiMAX network during a listening interval, using a power saving class (PSC) that the MS initiates to prevent power consumption.
Conventionally, the MS may communicate in Wi-Fi intermittently using the power saving class of the broadband wireless access system. However, co-existence mode period allocation based on the power saving class is classified according to the characteristics of IEEE 802.16 data traffic. For Wi-Fi traffic transmission in co-existence mode, it is efficient to allocate a co-existence mode period suitable for the characteristics of Wi-Fi transmission traffic. Accordingly, there exists a need for efficiently allocating frames, taking into account the transmission duration or transmission time of Wi-Fi data, etc. when an IEEE 802.16m BS allocates a Wi-Fi data transmission period to a multi-radio MS with two interfaces (i.e. an IEEE 802.16m interface and a Wi-Fi interface) in an IEEE 802.16m system using a subframe structure.